1. Field of the Invention
The present invention relates to a method and an apparatus for producing a magnetic recording medium.
2. Description of the Related Art
Accompanying with the increase of density of magnetic recording media in recent years, recording track has become extremely narrow. As a result, the dimensional stability of media themselves has been strongly desired. Particularly, this trend is more conspicuous in floppy disc type media in which symmetry is demanded in a circumferential direction.
Regarding to this kind of technique, Unexamined Japanese Patent Publication (kokai) Nos. 6-28668, 7-7507, 9-44846 and the like have been known.
Unexamined Japanese Patent Publication No. 6-28668 suggests conveying a tape-shaped medium at low tension with blowing hot air to the tape after coating process and calendering process for the purpose of heightening the dimensional stability of the medium and ensuring the productivity.
Unexamined Japanese Patent Publication No. 7-7507 discloses to increase dimensional stability for stabilizing the heat shrinkage of a floppy disc by heat-treating a medium punched into a doughnut-shaped pattern (called a cookie).
Unexamined Japanese Patent Publication No. 9-44846 discloses a technique of blowing hot air parallel to a web after film formation for smoothing with a view to preventing wrinkles of deposited film to thereby effect coating of a protecting layer in succeeding process.
However, the heat treatment method disclosed in Unexamined Japanese Patent Publication No. 9-44846 has a problem in the productivity. After a magnetic recording medium has been coated and dried, the web travels through winding-up process, the wound web is delivered and subjected to calendering process and winding-up process, the wound web is delivered and subjected to heat treatment and winding-up process. Thus, delivery and winding-up are repeated while the web is traveling through three processes. That is, the web is processed in processes of [deliveryxe2x86x92coating windingxe2x86x92up]xe2x86x92[deliveryxe2x86x92calendering processxe2x86x92winding-up]xe2x86x92[deliveryxe2x86x92heat treatmentxe2x86x92winding-up]. Therefore, wrinkles or the like are generated in the vicinity of the core of the web in each process, and a part of wrinkles is cut and discarded in the next process. Loss of the web increases due to this repetition. Further, a failure occurring due to winding-up results in the reduction of yield.
On the other hand, as to tape type media, the technique disclosed in Unexamined Japanese Patent Publication No. 6-28668 has effected the removal of heat shrinkage but some problems to be solved have been revealed to cope with higher density and disc type media in recent years. That is, although the technique is advantageous in that processing can be performed continuously and within extremely short period of time, this technique is not suitable to solve the anisotropy of dimensional stability. This is due to the mechanism that the stress relaxation in the transverse direction (TD) progresses extremely rapidly compared with that in the machine direction (MD) when a web is transported with applying a tension of about 0.15 kg/m. As a result, the balance of dimensional stability of the web is lost.
As a matter of course, the above collapse of balance is a serious problem not only to disc type media but also to other media such as high density tapes of late years where the accuracy of tracking in the transverse direction is required.
The technique of performing heat treatment in a cookie state as disclosed in Unexamined Japanese Patent Publication No. 7-7507 is good as technique itself but there is a problem in the productivity. That is, for the processing condition of from 60 to 80xc2x0 C. for 5 hours or more as shown in the technique, large productivity cannot be expected. Therefore, in general, a plurality of cookies are to be piled and subjected to heat treatment. However, there are risks of uneven heat conduction due to piling, the deformation of cookies and the reduction of yield. Further, increasing the temperature for shortening the processing time has sometimes caused softening of the binder in the magnetic coating solution coated on media followed by the adhesion of coolies to each other, which leads to the reduction of yield.
Unexamined Japanese Patent Publication No. 9-44846 suggests conditions of heating a web with hot air along the machine direction as an effective heating means for improving dimensional stability of a magnetic recording medium but according to this technique, same as above, yield is deteriorated due to the repetition of delivery and winding-up.
It is an object of the present invention to solve the above-described problems in the conventional art, and to provide a method for producing a magnetic recording medium capable of improving dimensional stability of a magnetic recording medium without reducing the yield and an apparatus therefor.
It is another object of the present invention are to solve the above-described problems in the conventional art, and to provide a method which is capable of producing a magnetic recording medium having a low heat shrinkage factor and small anisotropy by adjusting the tension and heating conditions properly.
According to the first aspect of the present invention, in a method for producing a magnetic recording medium comprising at least one magnetic layer formed on a nonmagnetic support by coating, after all the coating processes have been completed, heat treatment is performed by heat-treating said support which has undergone all the coating processes at 95 to 140xc2x0 C. for 1.0 to 10.0 seconds with continuously applying tension of from 1.0 to 4.0 kg/m, and then calendering process is performed at a temperature lower than the temperature of the foregoing heat treatment.
According to the second aspect of the present invention, an apparatus for producing a magnetic recording medium having at least one magnetic layer formed on a nonmagnetic support by coating comprises at least one coating part for performing all the coating processes including at least one magnetic layer of said support, a heat treatment unit for performing heat treatment of said support after having been coated with said magnetic layer with applying tension of from 1.0 to 4.0 kg/m at from 95xc2x0 C. to 140xc2x0 C. for from 1.0 to 10.0 seconds, and a calendering process part for performing calendering process at a lower temperature than the temperature of said heat treatment.
According to the third aspect of the present invention, an apparatus for producing a magnetic recording medium having at least one magnetic layer formed on a nonmagnetic support by coating comprises a coating part for coating said magnetic layer on said support, a drying zone for drying said coated magnetic layer, a heat treatment part for heat-treating said support after said magnetic layer has been dried, and tension setting means for independently setting the tension of said support at each processing part.
According to the fourth aspect of the present invention, a method for producing a magnetic recording medium comprises the steps of forming a magnetic layer on a nonmagnetic support to form a web of a magnetic recording medium, and heat-treating the traveling web of the magnetic recording medium, wherein the temperature of the web is in the range of 120xc2x0 C. to 220xc2x0 C., and the tension applied to the web is in the range of 0.05 kg/m to 3.0 kg/m.
According to the fifth aspect of the present invention, in the method for producing a magnetic recording medium of the fourth aspect, said web of the magnetic recording medium is a web of a disc-type medium.
According to the sixth aspect of the present invention, in the method for producing a magnetic recording medium of the fourth or fifth aspect, said heat treatment is heating by hot air.
According to the seventh aspect of the present invention, in the method for producing a magnetic recording medium of the fourth or fifth aspect, at least a part of a heating means is an infrared ray heating means.
In the present invention, the web which the coating and calendering has been completed is heat-treated under extremely low tension and high temperature. Thus, dimensional stability can be improved in a well-balanced state of anisotropy. Processing is more preferably performed at heating temperature of the web in the range of about 120 to 170xc2x0 C. for 1 to 10 seconds. The tension is preferably in the range of 0.05 kg/m to 3.0 kg/m, but when higher anisotropy is required, it may be 0.05 kg/m or less, if possible.